Antenna assembly with de-icing means



y 1955 c. E. THOMAS, JR., ET AL 2,712,604

ANTENNA ASSEMBLY WITH DE-ICING MEANS 3 Sheets-Sheet 1 Filed July 26, 1951 ll .lll

IN VENTOR-S 9 iv imumlrplmin :17 in Illllllllllliil 5.?

ATTORNEY y 5, 1955 c. E. THOMAS, JR, ET AL 2,712,604

ANTENNA ASSEMBLY WITH DE-ICING MEANS 3 Sheets-Sheet 2 Filed July 26, 1951 5/7/VFO/P0 A ma #7540 BY We ATTORNEY y 5, 1955 c. E. THOMAS, JR, ET AL 2,712,604

ANTENNA ASSEMBLY WITH DE-ICING MEANS Filed July 26, 1951 :s Sheets-Sheet :s

:: 70/1014 I 3.9 #6207 -1/1? *mfia.

N VENTORS BY Mafi NEY United States Patent Cfiice 2,712,604 Patented July 5, 1955 ANTENNA ASSEMBLY WITH DIE-ICING MEANS Charles Edgar Thomas, In, and Sanford Hershfield, Baltimore County, Md, assignors to The Glenn L. Martin Company, Middle River, Md., a corporation of Maryland Application July 26, 1951, Serial No. 238,686

3 Claims. (Cl. 250-33) This invention relates to an improved antenna assembly incorporating de-icing means for insuring eificient and reliable operation thereof under adverse weather conditions.

It is frequently necessary to locate radio or radar beacon transmitting or receiving stations in remote areas where severe weather conditions are encountered. The antennas for such stations are particularly severely affected by such conditions, especially where ice forms thereon which tends to seriously decrease the gain or efficiency of the antenna and to radically alter the propagation pattern of the antenna.

It is therefore an object of this invention to provide an antenna assembly incorporating an antenna of predetermined radiation characteristics, wherein the antenna is fully protected from the elements by a radome housing and wherein the radome is provided with de-icing means for preventing the accumulation of ice thereon.

A further object is to provide an effective heating means for such a radome, wherein the heating means will have negligible effect on the radiation pattern of the antenna.

Another object is to provide a fully automatic control means for starting and stopping the operation of the de-icing means.

A further object is to provide a novel antenna and feed arrangement therefor by means of which the radiation pattern will remain substantially constant over a relatively wide range of radio frequencies.

Further objects and advantages will become apparent from the following description and claims taken in conjunction with the accompanying drawings.

In the drawing:

Figure l is a side elevation of the complete antenna assembly with a portion of the radome broken away to show the antenna element.

Figure 2 is a front view of the assembly with parts broken away to show the interior.

Figure 3 is an enlarged vertical sectional view taken through the mid-portion of the antenna assembly.

Figure 4 is a similar view showing the lower portion of the assembly.

Figure 5 is a transverse section taken on the line 55 of Figure 4.

Figure 6 is a sectional view taken on the line 66 of Figure 4.

Figure 7 is an enlarged fragmentary view of the radome assembly with parts broken away to show the de-icing element and showing in schematic form the automatic control means for the de-icing unit.

The antenna assembly 1 comprises a slotted waveguide antenna 2 housed within an elongated cylindrical radome 3 and arranged to be excited by means of a conventional waveguide 4. The assembly includes a base 5 adapted to be attached in any desired fashion to appropriate supporting structure. Rigidly carried by the base 5 is a vertically extending hollow cylindrical support 6 which,

in turn, serves to support the antenna and radome assembly. Also carried by the support 6 is a housing 7 within which is mounted the control unit (to be later described) for the de-icing means associated with the radome and to which electrical energy is conducted by suitable conductors housed within cable 8. A capacity type ice sensing means 9 is supported adjacent the lower end of the radome 3 and serves, as will be later described in detail, to control the operation of the de-icing means.

The antenna unit itself is formed from a thin pancake waveguide section having a plurality 'of elongated slots 10 extending through the front and back walls 11 and 12. As clearly shown in Figure 2, the slots are so arranged as to form upper and lower radiating arrays 13 and 14, each consisting of, in the particular case, 10 slot pairs. The number of slots provided may, of course, be varied without affecting the principle of operation of the antenna, it beihg well understood in the art that the number of slots employed depends upon the beam width desired in the vertical plane.

As previously mentioned, energy is supplied to the antenna from the waveguide feed section 4, the portion 15 of which'adjacent the antenna section tapers in thickness as shown in Figure 6 until it has the same internal dimensions as that of the antenna section 2. Direct communication between the waveguide feed section and the antenna 2 is prevented by a block 16 forming a short circuit across the waveguide and serving as the lower end wall of the lower array 14. The portion 17 of the waveguide between block 16 and tapered section 15 is arranged as a single ended transition section of the doorknob" type, by means of which energy within the waveguide may be transferred to a co-axial line 18 which leads upwardly along one edge of the lower array within a suitable hollow housing 19 secured along and forming the corresponding edge of the antenna unit 2. As best shown in Figure 5, the opposite edge 20 of the antenna section 2 is thickened and contoured so as to conform in outer configuration to the contour of housing 19, so that the circularity of the horizontal radiation pattern will not be adversely affected.

Co-axial line 18 is connected to a second doorknob unit 21 which forms a portion of a double ended transition section 22 located between the upper and lower arrays. Suitable irises 23 extend into the interior of the Waveguide to match the impedance of the arrays to that of the transition section 22, While the axis of the doorknob 21 is offset laterally of the longitudinal axis 23' of the waveguide so as to provide a proper impedance match between the co-axial line 18 and the transition section 22. Similar irises 24 (see Figs. 4 and 6 are also provided adjacent the transition section 17 to insure a proper impedance match between the tapered waveguide 15 and the co-axial line 18.

The instant antenna is intended to operate over a relatively wide range of frequencies and to produce a substantially circular radiation pattern in the horizontal plane and a narrow beam in the vertical plane. Moreover, it is desired to maintain the direction of maximum propagation in the vertical plane at a substantially constant predetermined upward angle (about 1) relative to the horizontal. It is well known that the angle of maximum propagation from such an array, measured in the vertical plane, will depend upon the phase relation ship of the excitation applied to successive slots along the array, and that any shift in frequency will therefore cause a corresponding and normally undesired shift in the angle of maximum propagation. By providing two spaced co-linear arrays 13 and 14, and by feeding them at a point between the arrays, applicants have substantially eliminated any variation in the angle of maximum propagation due to the change in frequency within the design limits. With the arrangement shown,

a change of frequency which will cause the direction of maximum propagation from the lower array to shift upwardly for example, will cause a corresponding downward shift in the direction of propagation from the upper array. Since the propagation fields from the two arrays combine to give the overall propagation field for the antenna assembly, the individual shifts in directivity are effectively cancelled out with the result that the direction of propagation from the complete assembly remains at a substantially fixed angle relative to the horizontal through the desired range of frequency.

Moreover, by oifsetting the feed point for the antenna (which feed point is defined by the axis of the doorknob 21) slightly below the middle of the antenna (indicated by the line 25 in Figure 3), the direction of maximum propagation from the assembly is can to lie along an angle of slightly more than l above the horizon.

Tests of an antenna constructed as described above for operation over the frequency range from 9220 me. to 9430 me. have shown the half-power beam width in the vertical plane to remain between 3.0 and 33 throughout the frequency range while the angle of maximum radiation in the vertical plane remains between 105 and 130 above the horizontal. Thus, despite the relatively wide range of frequencies involved, there is only 0.3 variation in beam width and only 0.25 variation in directivity in the vertical plane.

ln order to insure efficient operation of the antenna despite adverse weather conditions, tne radome 3 is made air-tight and is provided with de-icing means. The radome consists of a generally cylindrical body built up from laminations of melamine Fiberglas and is provided with a flange 24 formed integrally therewith and secured by collar 27 to the upper end 28 05 the cylindrical support 6. The upper end of the radome is secured as shown in Figure 2 to the upper end of the antenna unit 2, suitable Oring seals 29 being provided at both ends to make the assembly airand moisturetight. As indicated in Figure 6, a small hole 3') is provided in the rear wall of the waveguide so that the entire assembly may be suitably pressurized from the waveguide 4. Pressure is also supplied to the interior of the housing 7 by means of a suitable conduit 30' (Fig. l).

Mounted between the outer laminations of the radorne 3 is a continuous high resistance wire 31, arranged as shown in Figure 7 so as to provide a plurality of circumferentially spaced. vertically extending heating elements 32 extending substantially the full length of the radorne. By having these heating elements extend vcrtically, they are perpendicular to the E-vector of the electromagnetic field radiated by the slots 10, thus having a negligible effect upon the radiation characteristics of the antenna and, at the same time, minimizing the absorption of energy. The ends of wire 31. are connected to the conductors within cable 8, which in turn, lead to the normally open contact points 33 of a heater control relay 34.

Relay 34 forms a part of an automatic de-icing control unit housed within housing 7. This control unit includes an oscillator 37 having a balanced output which is applied across a capacity bridge, one leg of wh1ch is formed by the capacity sensing unit 9 and the other by a balancing condenser 36. When no ice is present on the sensing unit 9 the bridge is balanced so that there is no output therefrom. However, when a predetermined quantity of ice has accumulated on the unit 9, the bridge is thrown out of balance and the resulting voltage at 37 is rectified by the rectifier 33 and applied to the control grid of a suitable amplifier 39. Amplifier 39 is thereupon made conductive and energizes control relay 34 so as to cause the latter to close its points 33 and connect the heater element to a suitable source of electrical energy.

The capacity type sensing unit 9 is suitably supported as shown adjacent the lower end of the radome and comprises a plurality of radially extending plates 40, alternate plates being electrically connected together so as to form a multiple plate condenser assembly. Leads 41 and 42 serve to connect the two sets of plates to the rest of the control circuit as shown in Figure 7. Ice accumulating on the sensing unit causes a change in capacity, which, as previously described, is detected by the control unit to initiate the operation of the heater. Since the unit 9 is closely adjacent the lower end of the radome, as soon as the ice is melted from the latter. sufiicient heat will be radiated to the sensing unit to similarly melt the ice therefrom, once again restoring the capacity bridge to a balanced condition and terminating the operation of the heating means.

In use, the radome 3 and the de-icing control housing 7 will be pressurized by way of the feed waveguide 4 so that moisture, dust, etc., will be elfectively prevented from getting into the interior thereof. The radomc is rigidly supported so that it will readily withstand strong winds and protect the antenna element therefrom. The dc-icing means will permit effective use of the antenna even under severe icing conditions.

Radio energy travelling along the interior of the waveguide 4 will be transferred at 17 to the co-axial line 18, from the line 13 to the transition unit 22, by means of which it will be simultaneously applied to the upper and lower arrays so as to excite the slots therein in the proper phase relationship. As previously discussed, the feed point 21 being slightly below the center of the antenna, the phase of the exciting waves at the slots of the lower array will always be advanced slightly with respect to that in the upper array, resulting in a slight upper tilt to the transmitted beam. Since waveguide of relatively thin section is used for the antenna portion and since slots are formed in both the front and back wall thereof, a substantially circular radiation pattern will be obtained in the horizontal plane. By thickening the edge walls and housing the co-axial line 13 therein, the symmetry of the pattern is maintained.

Despite the small dimensions of the radome and the resultant close proximity of the heating elements imbedo'ed therein to the antenna itself, the arrangement of thse elements perpendicular to the polarization of the radiated energy results in but negligblc distortion of the field pattern and/or absorption of energy therefrom.

\Nhile, for ease of description, the antenna has been shown as vertically arranged. it could obviously be differently oriented without affecting its operation. Regardless of its orientation the patterns and polarization will have the same relationship to the longitudinal axis as in the described arrangement.

Other feeding systems could also obviously be substituted for that shown and described although the arrangement shown has been found to be particularly effective and of relatively simple construction.

While described as being used for transmitting, no limitation is intended thereby as it is obvious that the antenna is equally useful for receiving.

Many other changes and substitutions could obviously be made without departing from the scope and spirit of the invention as defined by the appended claims.

We claim as our invention:

1. An antenna assembly comprising an elongated slotted waveguide antenna, said waveguide antenna having the slots therein spaced longitudinally therealong and arranged to form two slot arrays disposed symmetrically with respect to the central portion of said antenna, and means for coupling clcctro-magnctic energy to said antenna at a point between said arrays comprising a coaxial transmission line extending lengthwise along one edge of said waveguide, a housing for said co-axial line extending along and secured to said edge, and the opposite edge of said waveguide being formed to conform in con tion of waveguide, means forming a short circuit across.

said waveguide intermediate its length, and dividing said waveguide into a feeding section and an antenna section, said antenna section being provided with a plurality of radiating slots spaced symmetrically along said antenna section with respect to the central portion thereof whereby to form two separate arrays, and means for coupling electro-rnagnetic energy from said feeding section to said antenna section comprising a transmission line extending along said antenna section and having its ends coupled respectively to said feeding section adjacent said short circuit forming means and to the central portion of said antenna section at a point between said arrays. 5

3. An antenna assembly comprising an elongated section of waveguide, means forming a short circuit across said waveguide intermediate its length, anddividing said waveguide into a feeding section and an antenna section, said antenna section being provided with a plurality of radiating slots spaced symmetrically along said antenna section with respect to the central portion thereof whereby to form two separate arrays, and means for coupling electro-magnetic energy from said feeding section to said antenna section comprising a transmission line extending along said antenna section and having its ends coupled respectively to said feeding section adjacent said short circuit forming means and to the central portion of said antenna section at a point between said arrays, said last mentioned coupling point being located closer to one of said arrays than to the other whereby the direction of maximum radiation from said antenna will form an acute angle with a plane perpendicular to the axis of said antenna.

References Cited in the file of this patent UNITED STATES PATENTS 1,404,726 Alexanderson Jan. 31, 1922 2,008,266 Sterba July 16, 1935 2,115,787 Runge May 3, 1938 2,243,677 Lindenblad (2) May 27, 1941 2,298,272 Barrow Oct. 13, 1942 2,414,266 Lindenblad Jan. 14, 1947 2,426,976 Taulman Sept. 2, 1947 2,451,258 Trevor Oct. 12, 1948 2,513,007 Darling June 27, 1950 2,562,332 Riblet July 31, 1951 2,573,746 Watson et al Nov. 6, 1951 2,617,031 Bolljahn Nov. 4, 1952 FOREIGN PATENTS 944,305 France Apr. 1, 1949 

